When designing and manufacturing microwave equipment including low noise amplifiers, the active element, such as a field effect (FET) transistor, must be properly connected to a circuit that presents the optimum source and load impedance to its input and output terminals. Some means must be provided to alter these impedances to compensate for variations in the FET's and in specific circuit dimensions. The higher the frequency involved in the microwave circuit, the more critical these variances become. For microwave low noise amplifier (LNA) cirCuils that use GaAs MESFET transistors on microstrip circuits, some means must be provided to trim the matching circuit geometry at the transistor terminals to compensate for changes in the optimum source and load impedances of the transistors from different lots. One method of accomplishing this tuning and transistor characteristic matching may be found in a patent granted to one of the present inventors and assigned to the same assignee as the present invention and given U.S. Pat. No. 4,472,690.
Some microwave circuits using ceramic substrates are designed with the FET devices mounted to the ground plane using flanges that may be bolted to a metal plate. Other FET devices are physically mounted using solder type attaching mechanisms. However, in either case, the ceramic substrate that is connected to the FET must be soldered to a common ground plane to ensure intimate contact of the substrate ground plane with the FET ground flange to the chassis ground plane. The impedance matching stubs for a typical FET are etched or plated on a dielectric substrate causing them to present fixed impedances to the FET. When a FET is replaced by a different FET, the different noise parameters require that the impedance matching stubs be retuned using suitable metal or dielectric chips. The tuning operations can be expensive and require trained personnel to carefully assemble and test the amplifiers.
Although the above approaches have worked at frequencies of up to six gigahertz, the problems associated with the prior art approach are amplified even further as the frequency is extended beyond six gigahertz.
The solution of the present invention is to incorporate the compensation necessary to tune the active components on a unitary structure, which can be easily tested and adjusted to have a standardized set of characteristics in spite of variations in individual active and passive components, producing the assemblage to simplify testing and trouble shooting of the remaining circuitry.
It is thus an object of the present invention to provide a low noise amplifier module.